Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	$ 2,031.1	$ 1,107.3	$ 1,336.0
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Accrued Income Taxes	183.5	(44.4)	61.3
Other Operating Activities, net	(41.6)	774.5	(333.7)
Net Cash (Used in) Provided by Operating Activities	(486.0)	2,625.6	2,392.4
CASH FLOWS FROM INVESTING ACTIVITIES
Other Investing Activities, net	633.2	(1,539.0)	(319.3)
Net Cash Provided by (Used in) Investing Activities	(2,563.5)	4,784.1	25,929.8
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes	0.0	0.0	1,988.8
Repayments of Senior Notes	0.0	0.0	(500.0)
Treasury Stock Purchased	(937.8)	(347.5)	(35.4)
Net Proceeds from Stock Options	9.4	2.3	3.9
Cash Dividends Paid on Common Stock	(602.3)	(621.5)	(750.2)
Cash Dividends Paid on Preferred Stock	(41.8)	(41.8)	(46.5)
Other Financing Activities, net	(6.4)	(2.7)	0.0
Net Cash Provided by (Used in) Financing Activities	3,439.5	(7,182.6)	(26,437.4)
Change in Cash and Due from Banks	(114.3)	137.3	1,597.4
Cash on Deposit with Subsidiary Bank at Beginning of Year	4,791.5
Cash on Deposit with Subsidiary Bank at End of Year	4,677.2	4,791.5
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	2,031.1	1,107.3	1,336.0
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Equity in Undistributed Net Income of Subsidiaries	1,101.5	(284.3)	(1,356.2)
Change in Prepaid Expenses	1.4	1.7	(3.8)
Change in Accrued Income Taxes	(92.9)	(10.2)	36.6
Other Operating Activities, net	118.9	138.1	(608.2)
Net Cash (Used in) Provided by Operating Activities	3,160.0	952.6	(595.6)
CASH FLOWS FROM INVESTING ACTIVITIES
Investments in and Advances to Subsidiaries, net	0.0	250.0	(1,200.1)
Other Investing Activities, net	0.1	0.0	(0.1)
Net Cash Provided by (Used in) Investing Activities	0.1	250.0	(1,200.2)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes	0.0	0.0	1,988.8
Repayments of Senior Notes	0.0	0.0	(500.0)
Repayment of Floating Rate Capital Debt	0.0	0.0	0.0
Treasury Stock Purchased	(937.8)	(347.5)	(35.4)
Net Proceeds from Stock Options	9.4	2.3	3.9
Cash Dividends Paid on Common Stock	(602.3)	(621.5)	(750.2)
Cash Dividends Paid on Preferred Stock	(41.8)	(41.8)	(46.5)
Other Financing Activities, net	0.0	0.0	4.9
Net Cash Provided by (Used in) Financing Activities	(1,572.5)	(1,008.5)	665.5
Change in Cash and Due from Banks	1,587.6	194.1	(1,130.3)
Cash on Deposit with Subsidiary Bank at Beginning of Year	795.5	601.4	1,731.7
Cash on Deposit with Subsidiary Bank at End of Year	$ 2,383.1	$ 795.5	$ 601.4